[libcxx-dev] Unqualified name use causes ambiguity through ADL

Marcin Zalewski via libcxx-dev libcxx-dev at lists.llvm.org
Fri Oct 11 21:11:59 PDT 2019

Thanks Eric. I guess this could use a bug report. I searched existing bug
reports, but I did not find anything about it. Are you involved in
development of libc++?

Thank you,

On Fri, Oct 11, 2019 at 6:28 PM Eric Fiselier <eric at efcs.ca> wrote:

> On Wed, Oct 9, 2019, 12:40 AM Marcin Zalewski via libcxx-dev <
> libcxx-dev at lists.llvm.org> wrote:
>> I have a library that redefines forward_as_tuple in its namespace, along
>> with some concrete types. This causes the problem with the following code:
>> https://github.com/llvm/llvm-project/blob/master/libcxx/include/tuple#L1352
>> https://github.com/llvm/llvm-project/blob/master/libcxx/include/tuple#L1367
>> Here, forward_as_tuple is being called unqualified, and ADL kicks in. ADL
>> finds the definition of forward_as_tuple in the library that I am using,
>> and I get ambiguity.
>> It definitely can be argued that the library I am using makes a dangerous
>> choice by sticking concrete types into the same namespace as the template
>> forward_as_tuple, but it does. My question is why libc++ calls
>> forward_as_tuple unqualified rather than std::forward_as_tuple. Is this a
>> bug, or is there a good reason for this?
> This is a bug in libc++.
> Apart from customization points like `swap`, libc++ should never use ADL,
> even when calling functions with reserved names.
>  I'm working on a Clang tool to clean up ADL-using call expressions. I'll
> run it over libc++
>> Thanks,
>> Marcin
>> _______________________________________________
>> libcxx-dev mailing list
>> libcxx-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev
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